One example of a control loop is a phase-locked loop. A typical phase-locked loop oscillator circuit has a variable frequency oscillator (VFO), such as a voltage controlled oscillator, a divider or prescaler for dividing the variable frequency output of the VFO down to a desired frequency or band, a phase comparator for comparing the phase of the frequency-divided VFO signal with the phase of a reference signal, which may also be frequency-divided, and a loop filter for low pass filtering the output of the phase comparator so as to provide an error signal to the VFO, thereby causing the VFO to achieve the desired oscillation frequency. The reference signal may be a locally generated reference, such as the output of a crystal oscillator, or may be a remotely generated reference, such as a received phase, frequency or amplitude modulated signal. With the exception of the loop filter, the general design of a phase-locked loop oscillator is straightforward and well known in the art.
The design of a loop filter is a tradeoff between two opposing goals: lock time, and stability. The response time of a loop filter determines how quickly a function generator can lock to the reference signal when the reference signal or some other parameter changes. A fast response calls for a small time constant, and therefore a higher cutoff frequency, for the loop filter. Once lock has been achieved then the function generator should be very stable. However, a fast lock typically means that the function generator is very susceptible to noise. In most cases a compromise is made between these two divergent goals and the response time of the loop filter is set so that the output of the function generator is reasonably stable but the output of the function generator will lock to the reference signal within an acceptable time.
In one type of function generator, a VFO, the reference signal may change frequency, or the frequency divider ratio may be changed so as to generate a different VFO output frequency. It is generally desirable, especially when the reference signal is remotely generated, to cause the VFO to quickly lock to the reference signal and to accurately and quickly track any changes in the frequency of the reference signal, hence a fast response time is desired. However, in some applications, such as a demodulator, it is desirable that the VFO frequency not precisely follow the reference frequency but only follow variations in the average of the frequency of the reference signal. These factors call for a loop filter with a slower response time, which means that a lower cutoff frequency is desired.
One approach to achieving both goals is to vary the response time of the loop filter. In the past, varying the response time for the loop filter has been accomplished by switching capacitors in and out of the loop filter or by changing the resistance in the loop filter. For example, see U.S. Pat. Nos. 3,657,661 to Jarger, 4,093,824 to Grosjean, and 4,479,091 to Yoshisato.
However, switching components into and out of a circuit can introduce noise and cause spurious output function changes, such as frequency shifting. Furthermore, although the theoretical resistance range of a semiconductor device is quite large, the useful range is significantly smaller. This range limitation has the effect of determining the value of the capacitor which must be used in the loop filter. Generally, the capacitor is quite large and expensive, thereby increasing cost and space requirements, and cannot be manufactured as part of an integrated circuit.
Therefore, there is a need for a loop filter which has a variable response time and which does not require switching components in and out of the loop filter. It would also be desirable to eliminate the need for a large value (and therefore typically large size and cost) discrete capacitor in the loop filter. This also allows the control loop to be constructed as an integrated circuit.
These same limitations apply to loop filters used in other circuits as well. For example, the loop filter in an automatic gain control circuit should have a response time which allows for compensation of normal signal strength variations but which does not respond so rapidly as to counteract the modulation caused by the information which is being transmitted. Likewise, the loop filter in an automatic frequency control (AFC) circuit must allow the AFC circuit to compensate for normal frequency and phase drifting and shifting but must not allow the AFC circuit to act so quickly as to remove the transmitted information.